A unified model for the 3.28 μ m and 3.4 μ m spectral feature in the interstellar medium and in comets

The 3.4 μ m absorption feature seen in the spectra of GC-IRS7 and also in other sources in the galactic centre area is shown to be of interstellar rather than circumstellar origin. The implication is that a large fraction of interstellar grains are organic in character. The 3.28 μ m feature seen in emission in planetary nebulae, in external galaxies, and in the diffuse infrared radiation from the Galaxy is most likely due to aromatic hydrocarbons which are released as degradation products of organic grains. The same aromatic molecules can also explain the 2200A interstellar extinction feature. The detailed 2.8-3.6 μ m profiles of IRS7 obtained from both AAT spectra and Japanese sources can be explained on the basis of grains comprised of material closely resembling biogenic matter in their disposition of CH linkages. Dust particles possessing the same detailed spectral characteristics as the interstellar grains are able to match infrared spectra obtained for Halley's Comet